why i cannot update my iphone 6 in 12.5.3 ?

Are you asking if you can update an iPhone 6 to iOS 12.5.3, or are you asking if you can update it past iOS 12.5.3? An iPhone 6 can run iOS 12.5.3, but that is the highest it will run. It does not have the hardware required to run any higher iOS.

What happened when you try, error messages?

You can update to iOS 12.5.3 provided:

• iPhone have enough free space (4-5 GB)
• iPhone is connect to the Internet using Wi-Fi (not Personal Hotspot)
• iPhone is connected to wall power (switch OFF Low Power Mode)
• iPhone is not using iOS beta